Environmental interests and an increasing worldwide demand for energy have encouraged energy producers to investigate renewable energy sources, including biofuels. Biofuel is obtained from biological material that is living or relatively recently lifeless, in contrast to fossil fuels (also referred to as mineral fuels) which are derived from ancient biological material. There is particularly interest in biofuels where, as in Europe, regulatory requirements have been or will be introduced that will require increased use of biofuels for motor vehicles, principally by blending with mineral fuels.
Biofuels are typically made from sugars, starches, vegetable oils, or animal fats using conventional technology from basic feedstocks, such as seeds, often referred to as bio-feeds. For example, wheat can provide starch for fermentation into bioethanol, while oil-containing seeds such as sunflower seeds provide vegetable oil that can be used in biodiesel.
Some diesel engines are compatible with pure biodiesel, without the need for modifications. But generally biodiesel is mixed with mineral diesel so that it may be used in a wider range of diesel engines. Currently vehicle manufacturers recommend use of fuel containing up to 15% biodiesel blended with mineral diesel.
The conventional approach for converting vegetable oils or other fatty acid derivatives into liquid fuels in the diesel boiling range is by a transesterification reaction with an alcohol, typically methanol, in the presence of catalysts, normally a base catalyst such as sodium hydroxide. The product obtained is typically a fatty acid alkyl ester, most commonly fatty acid methyl ester (known as FAME). While FAME has many desirable qualities, such as high cetane and its perceived environmental benefit, it has poor cold flow relative to mineral diesel because of its straight hydrocarbon chain. It also has lower stability because of the presence of ester moieties and unsaturated carbon-carbon bonds.
Hydrogenation methods are also known to convert vegetable oils or other fatty acid derivatives to hydrocarbon liquids in the diesel boiling range. These methods remove undesirable oxygen by hydrodeoxygenation to produce water, hydrodecarbonylation to produce CO, or hydrodecarboxylation to produce CO2. In hydrodeoxygenation, unsaturated carbon-carbon bonds present in feed molecules are saturated (hydrogenated) before deoxygenation. Compared to transesterification, this type of hydrotreating has the practical advantage that it may be practiced in a refinery utilizing existing infrastructure. This reduces the need for investment and provides potential for operating on a scale that is more likely to be economical.
There are methods, developed by UOP (EcoFining) and Neste, which process triglycerides, such as found in vegetable oils, in a stand-alone manner. For instance, PCT Publication No. WO 2008/020048 describes a process for coprocessing triglycerides with heavy vacuum oil in single or multiple reactors, and partial hydrogenation of oxygenated hydrocarbon compounds such as glycerol is disclosed as being more desirable from the perspective of hydrogen consumption. PCT Publication No. WO 2008/012415 describes a process for the catalytic hydrotreatment of a feedstock derived from petroleum, of the gasoil type, in at least one fixed bed hydrotreatment reactor, wherein up to about 30% by weight of vegetable oils and/or animal fats are incorporated into the feedstock, and the reactor is operated in a single pass without recycle.
European Patent No. EP 1911735 describes co-hydrogenation of a carboxylic acid and/or derivative with a hydrocarbon stream from a refinery, as a retrofit. CoMo or NiMo catalysts are disclosed. It is stated that conditions are maintained in the reactor such that almost complete conversion of the carboxylic acid and/or ester is achieved, that is, greater than 90% conversion and preferably greater than 95% conversion. The product is described as suitable for use as or with a diesel fuel.
PCT Publication No. WO 2008/040973 describes a process, which is suitable as a retrofit, in which a mixed feed of carboxylic acid and/or derivatives including esters, and a refinery process stream, such as a diesel fuel, are hydrodeoxygenated or simultaneously hydrodesulfurized and hydrodeoxygenated. The catalyst may be Ni or Co in combination with Mo. The process produces a product which is described as suitable for use as diesel, gasoline or aviation fuel. It is stated that, under the described conditions, conversions of greater than 90% of the co-fed carboxylic acid and/or derivatives are typical and usually greater than 95% is achieved.
PCT Publication No. WO 2007/138254 describes a process in which in a first stage a hydrocarbon process stream, which may be a middle distillate, is hydrogenated and then fed with a carboxylic acid and/or ester to a second hydrogenation stage. The final product may be diesel fuel, and the benefits are said to be reduced exotherm, improved diesel yield, reduced fouling, reduced coking, and reduced residual olefins and/or heteroatoms. Mention is made of an alternative process in which an untreated hydrocarbon process stream is fed with the ester. Conditions in the second reactor are said to be the same as the first, and NiMo and CoMo are described as preferred catalysts for the first reactor. It is stated that conditions are maintained in the reactor such that almost complete conversion of the carboxylic acid and/or ester is achieved, that is greater than 90% conversion and preferably greater than 95% conversion.
Unlike conventional distillate hydrodesulfurization, direct hydrotreating of vegetable oils or animal fats requires a relatively high amount of hydrogen and is generally accompanied by a large amount of heat release, which requires extremely careful control. Otherwise undesirable side reactions, such as cracking, polymerization, and aromatization may result. Additionally, co-processing triglycerides and FAME over CoMo catalysts has shown a hydrodesulfurization debit. Therefore, there is a need for an improved hydrotreating process for vegetable oils and animal fats, and preferably that may be performed in existing equipment for treating mineral fuels.